FI127386B - A process for steam generation at a pulp mill digester - Google Patents

Abstract

The present invention relates to a method of producing steam at a digester plant of a chemical pulp mill. Black liquor is discharged from the digester at a first temperature and pressure. The black liquor is treated in a flash tank for generating flashed black liquor and flash steam at a second temperature and a second pressure, which are lower than the first temperature and pressure. The flashed black liquor is taken for further treatment to the evaporation plant. The flash steam is led to an indirect heat exchange contact with clean liquid in the reboiler for boiling the liquid and for generating steam. The steam is led from the reboiler into a fan or compressor for increasing the steam pressure to a third pressure, which is higher than the second pressure and for adjusting the pressure of the steam in the flash tank. Steam at a third pressure is used for pretreating comminuted cellulosic fiber material, such as chips, prior to cooking.

Description

METHOD FOR THE DEVELOPMENT OF STEAM IN THE PULP MILL

The present invention relates to a process for generating steam from black liquor in a pulp mill digester.

The prior art employs a fiber line system with wood chips, in which wood chips or other cellulosic material is steamed and liquid is added thereto to form a slurry, followed by pressure (also referred to as a feed system), feed into a treatment vessel or vessels (which may be impregnation vessel, followed by a kettle (this part is also called a cooking system). Currently, at least one black liquor stream (typically 120-170Ό) is removed from the cooking system. The removed black liquor stream or streams are used as a heat source for the "preheating" of white liquor, other black liquor streams15 and / or other liquid streams to the feed and cooking systems. The removed black liquor stream (or streams) is then led to a pre-evaporation system, i.e. one or more flash tanks, where hot black liquor is vaporized as it cools, typically to a temperature of about 100-120Ό. At this point, the black liquor is introduced into the evaporator system of the recovery zone. The expansion steam thus produced can be used in another object in the pulping process. The expansion steam, for example, can be used to preheat the chips directly before cooking.

The expansion process described above, although successfully used in conventional steams, has the disadvantage that the steam produced contains volatile compounds, including sulfur compounds, which are undesirable for wood chips pre-steaming. Typically, wood chips are steamed at atmospheric pressure or slightly higher so that residual gases are not absorbed into the wood chips but collected and treated. The treatment is typically a combustion in a non-condensable gas (NCG) system. This collection and treatment system becomes particularly important when the vapor used contains volatile compounds, including sulfur compounds, which have an adverse environmental effect, including harmful odor. Therefore, it is preferable to use a steam source which minimizes or eliminates the introduction of volatile compounds into the evaporation process. In addition, there is a risk of explosion if concentrated gases are added to the dilute gases together with the expansion35 vapor.

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U.S. Patent No. 6,722,130 discloses a system for producing pure steam from black liquor. First, the pressure of the black liquor is lowered to provide a second black liquor with a higher concentration and black liquor vapor which is condensed. This condensate is heated with the first black liquor and the evaporation evaporated to produce pure steam for the chips silo.

US 6,176,971 discloses a system for producing pure steam for use in a chips silo. Substantially pure usable steam is generated from hot spent processing liquor (e.g., black liquor) by boiling the spent liquor and then pressurizing (e.g., by an eductor, blower, or compressor) this clean, boiler-removed steam.

Another process for producing pure steam is disclosed in US 6,306,252 for use in a chips silo, wherein the black liquor from the digester is passed through a heat exchanger to heat clean process water, followed by lowering the pressure of the heated process water to produce pure steam.

WO 2007073333 discloses a system and method for producing steam at a chemical pulp mill digester. In the first step, the pressure of the hot pressurized black liquor from the digester is reduced to produce black liquor vapor, which is used to pre-treat the chips in the second preheating step. The pure steam for pretreating the chips in the first preheating step is generated by reheating the black liquor, which was depressurized before the final subsequent pressure reduction, in which an increased amount of black liquor vapor is introduced into the steam converter to produce pure steam.

Fl 122983 discloses a method of removing a first stream of black liquor from a digester, which is evaporated using fresh steam as a heating medium. Evaporation generates the steam needed for the cooking process. The second black liquor stream is also removed from the digester 30, which is expanded, and the resulting expansion steam is led to a heat exchanger in contact with a heat exchange fluid to produce pure steam.

Fl 122534 relates to evaporation of black liquor, and how steam utilization is improved in connection with a vapor compression evaporator and a multi-stage evaporator. The fan / compressor is an integral part of a steam press evaporator known per se.

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Known solutions provide a variety of systems for producing cleaner steam for the heating needs of the stove and improving the energy economy of the pulp mill.

In the above solutions, it has often been desirable to avoid the use of expansion steam in a chip silo, since it causes odor problems and an explosion risk. Therefore, boiler (reboiler) solutions, such as a kettle boiler or a vapor reboiler, are preferred. The disadvantage of the boiler boiler is that it does not increase the dry matter content of the black liquor. The problem with the steam boiler is that in certain cases this system may not always be able to produce enough steam in the chips silo, but will have to use low pressure steam in the silo. This can be done by feeding fresh steam directly into the chips or boiler water into the chips and thereby into the black liquor.

In the steam boiler solution, the black liquor is first expanded and the expansion steam is led to a boiler where water is boiled to generate steam. There may be a lot of resistance to this system: in the piping, in the boiler itself, and in the chip silo. In this case, the pressure in the expansion tank remains relatively high (up to 100 kPa (g)). In this case, the temperature of the black liquor after expansion may be as high as 120 O. The expanded black liquor usually has to be cooled by a condenser (liquid / liquid) before pumping to the evaporation plant. The higher the pressure in the expansion vessel, the higher the temperature of the black liquor and the lower the dry matter, which results in more steam being consumed by the evaporator. In addition, it is not possible to produce enough steam in the boiler for the chips silo, but it is necessary to add low pressure steam there. The direct use of steam reduces the black liquor solids, both directly and indirectly, by increasing the liquid to wood (N / P) ratio, which increases the consumption of intermediate steam at the top of the digester (direct steam if steam / liquid phase digester).

One object of the present invention is to avoid the above-mentioned problems and to provide an improved and alternative method of producing pure steam for steam pre-treatment of wood chips by utilizing the heat of black liquor as efficiently as possible and reducing the use of fresh steam. Another purpose is to produce steam by means of black liquor so that the properties of the treated black liquor are improved for further treatment in the pulp mill recovery area. One purpose is also to improve the pulp mill's overall energy economy.

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The present invention relates to a process for producing steam at a pulp mill digester, wherein the process

(a) introducing a slurry of pretreated finely divided cellulosic fibrous material and liquid into a digester for producing cellulosic pulp;

b) treating the material in the digester with a treatment chemical to produce pulp and black liquor;

c) removing the black liquor from the digester at the first temperature and the first pressure;

d) treating the black liquor in the expansion vessel to produce expanded black liquor and expansion 10 steam at a second temperature and a second pressure lower than the first temperature and pressure;

(e) the expanded black liquor is further processed to the evaporator;

f) introducing the expansion steam into an indirect heat exchange contact in the boiler with a pure liquid to boil the liquid and produce steam, said boiling being at said second pressure and second temperature;

g) passing the steam formed in step f) from the boiler to a fan or compressor to increase the steam pressure to a third pressure greater than the second pressure and to control the vapor pressure in the expansion vessel in step d); and

(h) using steam at a third pressure to pre-treat the comminuted cellulosic fibrous material, such as chips, before step (a).

In step g), the vapor pressure of the blower or compressor and the vapor pressure of the expansion vessel are adjusted as necessary for the steam required for chip chipping.

According to the present invention, the vapor reboiler vapor discharge line is provided with a compressor or blower, whereby the black liquor can be expanded in the expansion vessel to near atmospheric pressure. In this case, the temperature of the black liquor is typically just above 100 Ό (typically about 10 6 Ό) and the pressure in the expansion vessel is about 20 kPa (g). According to the present invention, operating a compressor or blower in a boiler outlet reduces pressure in the expansion vessel and boiler. Increasing the pressure applied to the chips treatment typically allows for the expansion evaporation of the hot liquids with a lower vapor temperature and higher vapor volume than in prior art processes.

20145212 prh 09 -03-2018 by means of the present invention reduces at least 5 ° C and preferably at least 10 ° C compared to prior art (without compressor / fan). The amount of gas (vapor) produced at the same time can typically be increased by at least 10%, preferably by at least 20% and in some cases by more than 40% compared to prior art.

The benefit of the present invention is most apparent when the cooking temperature is low (the temperature of the black liquor leaving the digester is low). This is typical, for example, when the pulp is cooked in the digester to a high kappa number (typically kappa number for more than 50 softwoods and more than 25 hardwoods). The temperature of the black liquor may be smoother than usual even when production is low in relation to the volume of the digester. Hardwood is cooked today at low cooking temperatures.

The solution of the invention is also advantageous when, in cold weather conditions, such as winter, the chips are cold, even icy, and thus the steam consumption is high for heating the chips. In winter conditions, expansion steam is not sufficient to heat the chips, but fresh low pressure steam must be added to the chips silo in current process solutions.

Although the term expansion vessel, which is also used by those skilled in the art, is used in this description, one skilled in the art will also know that any device for lowering the pressure of a hot pressurized fluid and allowing the liquid to evaporate, typically very rapidly, in a sealed container at a lower temperature. to produce

Drawing description:

Figure 1 schematically shows a system according to the invention.

Detailed Description of Preferred Embodiments

Figure 1 illustrates a system for generating steam at a stove and for treating black liquor so as to improve the temperature of the cooking process. The fiber line system comprises a chip silo 10, where wood chips or other cellulosic material introduced from line 11 are steamed with steam from line 12. If necessary, fresh steam can be introduced through line 12 '. The chips silage exhaust gas in line 50 is treated in a separator 49 to remove fines such as sticks and sawdust.

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Through line 13 (white liquor) and line 14 (return from the digester) the liquid is added to form a slurry, after which the slurry is pressurized by pumps 15 (this section is also referred to as a feed system). Then comes the spout 17 (this part is also called the cooking system). Prior to the boiler, the suspension may be optionally fed to a treatment vessel or vessels for treatment (there may be an impregnation vessel, a pre-hydrolysis process or other vessels, not shown). The suspension of chips and cooking fluid is fed along line 16 to the top of the digester 17, where steam is also added 33. Only the components that are important to the invention are shown in the drawing, and the cooking system may naturally include other types of steaming or feeding circuits or chips. The digester 17 includes at least two or more screen arrangements 18, 19 and 20. The digester includes a circulation line 21 associated with the screen 19, and the liquor is removed from the screen 19 and the liquor is added through line 22 and lye through line 23. Line 21 also has a heater 24. The filtrate from the filtrate is led in line 25 and further in lines 26 and 27 to the digester. The filtrate can also be added to the white liquor through line 28 and to the pulp to be removed from the digester in line 30 through line 29.

The hot black liquor removed from the digester, typically at a temperature of 120-160 Ό and a solids content of 12-17%, is removed through sieves 18 and 20 and passed along lines 32 and 33 to the expansion vessel 34. In the expansion vessel, black liquor pressure is can rise in the expansion. The expanded black liquor is sent along line 35 to the mill evaporator, where the black liquor is concentrated to a high solids content before being burned in a recovery boiler. The outlet line may include a heat exchanger 36 and a fiber filter 37.

The expansion steam is led along line 38 to a boiler 39 where its energy is recovered. The steam is subjected to an indirect heat exchange relationship with the pure liquid free of volatile compounds to warm it to a temperature above its boiling point to produce pure steam. Preferably, the boiler is a falling film tubular boiler. The expansion steam condenses on the inside of the tubes, while the pure liquid boils on the outside of the tubes to produce pure steam. The amount of steam produced increases when the side of the pure steam is vacuumed with a blower. Clean the steam-side pressure reduced to ensure that the black liquor may be more heat, which is itself released a greater amount of steam.

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The steam is discharged through line 12 which, according to an essential aspect of the invention, is provided with a pressure boosting device 41 which is a fan or a compressor. The steam is supplied to the blower from the boiler and the pressure is raised so that its pressure and temperature are sufficient to heat the chips in the silo 10. With the aid of a pressure-raising device, such as the blower, the amount of steam produced in the expansion tank can be increased.

The black liquor is typically pressurized in the digester to a pressure of 5 to 15 bar (abs) and its temperature corresponds to the cooking temperature (about 110 ° C to about 180 ° C). The invention is particularly suitable for cases where the temperature of the liquor removed from the digester is below

145 ° C. The expansion vessel 34 is of a substantially conventional construction and has an inlet 42 for a hot pressurized black liquor, an expansion evaporated steam outlet 44, and a cooled lower black liquor outlet 43. 1-4 bar (abs). Pai15 sun tanks are tanks designed for this purpose, which enable the pressure of hot pressurized black liquor to be reduced, whereupon the liquor evaporates very rapidly into liquid, typically water, thereby also increasing the concentration of the spent cooking chemical and cooking reaction products (solids). The steam produced by this rapid evaporation is removed from the vapor removal port 44 to a line 38 at a pressure of 34 in the tank 34, e.g. 100 ° to 140 ° C, sits at the bottom of the container 34 from where it is discharged from the outlet 43 along line 35. Typically, the remaining liquid is fed to an evaporator system.

Line 12 is used for steaming incoming wood chips. The pressure required for this steam is typically determined by the pressure requirements for the final use of the steam in the chipping of the chips. However, according to the present invention, increasing the pressure of the steam in line 45 by means of a fan 41 to the desired level of steam pressure (in line 12) allows the steam in line 45 to be produced at a lower pressure. In this case, the expansion vessel 34 and the boiler 39 can be operated at a lower temperature, whereby the expansion vessel and the boiler can, according to the invention, be able to produce more steam at a lower vapor heat 35. The temperature of the black liquor discharged from the expansion vessel to line 35 is also lower and the dry matter higher.

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A pressure boosting device, such as a fan 41, is typically speed-controlled or line-bladed. The fan is controlled by a pressure detector controller (not shown) connected to the fan, which receives a control signal from a pressure sensor located in the expansion tank 34. The steam pressure in the tank 34 and boiler 39 is typically the pressure of the steam supplied to the fan. As the need for steam in the chips silo increases / decreases, respectively, the fan speed is increased / decreased. Correspondingly, the temperature and dry matter content of the black liquor to the evaporator can be adjusted, in particular to increase the dry matter content.

The pure liquid is passed through line 40 to a boiler 39. It may typically be condenser condenser, demineralized water, boiler feed water, or a sufficiently pure water fraction such as factory hot water. The pure steam produced in the boiler contains substantially less non-condensable gases than the steam produced by direct black liquor expansion. Preferably, the pure steam of line 45/12 is used to preheat the wood chips with steam, e.g., in a chip silo 10. When this steam is used to treat the chips, less volatile compounds are generated and collected by the mill's NCG system since said vapor does not provide volatile compounds.

The expansion steam from the black liquor expansion tank contains volatile compounds such as sulfur compounds. These compounds get into the dirty condensate and into the stream of concentrated noncondensable gases (CNCG) formed in the boiler. The dirty condensate is sent from boiling 25 via line 46 to the evaporator where it is treated in a manner known per se. The CNCG flow is led through line 47 to condenser 48.

Example:

The starting point is that the temperature of the black liquor entering the expansion vessel is 136 ° O.

The pressure in the expansion vessel is 100 kPa (g). The temperature of the liquor leaving the tank is 121 Ό and the dry matter content is 14.7%. 225 kg / Adt of low pressure steam must be driven into the chips silo.

By installing a blower or compressor after boiling, the pressure in the expansion tank can be reduced to 20 kPa (g). The temperature of the outlet black liquor is 106 Ό and the dry matter content is 15.2%. The boiler produces enough steam for the chips silo and no low pressure steam is needed there.

The present invention has at least the following advantages:

• The need for fresh steam to heat the chips is reduced. In addition, the amount of steam can be easily adjusted by the fan / compressor to the amount of steam required.

• The total heat economy of the mill will improve as the dry matter content of the removed lye discharged to the mill's evaporator increases. The steam consumption and capacity requirements of the evaporation plant are reduced. The investment costs of the evaporator plant will be reduced in spring due to lower capacity requirements.

• Saving expensive so-called. boiler water • Reduce odor nuisance (compared to expansion steam use) • Reduce explosion risks (compared to expansion steam use).

Claims (5)

claim Procedure for generating meadow in the pulp mill's cookery, by which method a) a suspension of pretreated comminuted cellulosic fibrous material and liquid is passed to a digester (17) to produce cellulosic pulp; b) the material is treated in the digester with processing chemical to produce pulp and black liquor; c) the black liquor (32, 33) is removed from the digester (17) at a first temperature and at a first pressure; d) the black liquor is treated in an expansion vessel (34) to produce expanded black liquor (35) and the expansion bed (38) at a second temperature and at a second pressure lower than the first temperature and the first pressure, respectively; e) the expanded black liquor (35) is passed on to an evaporation plant; f) the expansion steam is led to indirect heat exchange connection with pure liquid in a boiler (39) to boil the liquid and generate meadow, said second pressure and other temperature rising in the boiler, characterized in that g) the meadow (45) produced in step (f) is passed from the cooker (39) to a fan (41) or compressor to raise the meadow pressure to a third pressure higher than the second pressure, and to adjust the meadow pressure in the expansion vessel ( 34) in step d) and h) the mud (12) at the third pressure is used for pre-treatment (10) of finely divided cellulosic fibrous material, such as wood chips, before step a). 20145212 prh 12 -03- 2018 Method according to claim 1, characterized in that the pressure (45) of the meadow (45) which is removed from the fan (41) or the compressor in step g) and the pressure of the meadow in the expansion vessel (34) is adjusted according to the meadow requirement at the pre-treatment (10). Fiber material, such as wood chips. 3. A method according to claim 1 or 2, characterized in that pulp (17) is produced with a high cutting number. 35 Method according to claim 1, 2 or 3, characterized in that the temperature of the black liquor (32, 33) removed from the digester (17) is below 145 ° C. Process according to any of the preceding claims, characterized in that it is desired to increase the dry content of the black liquor (35) conveyed to the evaporation plant.